Hitherto, if a shock or an unusual noise is generated upon shifting gears during traveling, it is thought that a gear clutch is abnormally worn, a transmission is taken out of a vehicle body, the abrasion of a clutch disc is measured, and, if it exceeds the regulation value, the clutch disc is considered to be worn out and it is replaced by a new one. Besides, even if such an unusual situation is not ascertained, the abrasion is measured as described above when the mileage or the working time reaches a predetermined value, and it is decided whether or not the clutch disc should be replaced by a new one. Accordingly, the abnormal wear of the clutch disc is not predicted before it occurs and is not recognized until the wear of the clutch disc actually progresses to a certain degree and an unusual situation occurs.
Hitherto, because the durations vary due to differences in working load condition even if the same type of clutch disc is used, one that may not have need of replacement may be overhauled and the clutch disc replaced as occasion demands. Furthermore, although the life of the clutch disc is designed on the supposition that it is used under the severest condition and is sufficient to endure to an overhaul, there is actually a case wherein the overhaul for inspecting the conditions of the clutch is not carried out until the clutch disc is worn in its earlier stage more than the regulation, and a slip, a vibration, or an unusual noise is generated, or it is damaged. For this reason, there is a problem in that an unexpected time is consumed, the work is late, and the working efficiency is lowered.
Besides, for example, between a case of a vehicle, such as a general motorcar or a dump truck which travels at a high speed, and a case of a bulldozer traveling at a low speed while fully driving a work machine, the transmission torques acting on the clutches vary. Therefore, the durability of the clutch disc for the vehicle such as the bulldozer, which runs at a low speed while fully driving the work machine, requires severeness more than the clutch discs of the general motorcar and the dump truck. The life of a construction machine is designed to be sufficient to endure to the time of an overhaul when it is used under a predetermined condition. Accordingly, in the construction machine in which the occasions of use at a predetermined high load are not many and which is used at a relatively light load condition, if an equal clutch disc is used, the life of the clutch disc becomes long and it is usable enough even when it is time for an overhaul. Contrarily, in the machine in which there are many cases wherein it is used at conditions higher than the predetermined high load, there is a problem in that it cannot endure to the time of an overhaul, and it is damaged.